1. Field of the Invention
This invention relates to improved adhesives and more particularly it relates to an improved starchbased borated corrugating adhesive modified with polyvinyl alcohol.
2. Description of the Prior Art
U.S. Pat. No. 2,102,937 discloses an adhesive composition comprising gelatinized starch, ungelatinized starch, an alkali, and borax in water. This mixture produced a level of adhesivity in the manufacture of corrugated paper board to form satisfactory bonds between the paper surfaces. The distinguishing feature of this invention was the use of ungelatinized starch. Under use conditions, the ungelatinized starch rapidly gelatinized under the heat and pressure at the point of formation of the corrugated board structure. The gelatinization resulted in a rapid increase in viscosity, and adhesivity of the adhesive composition. Borax was used in the composition to increase the level of viscosity during this final gelatinization step. This process revolutionized the corrugating industry. The basic process described in this patent and its companion case referred to below is still essentially the only process used to manufacture corrugated paper board to this date.
A companion case of the above, U.S. Pat. No. 2,051,025, is directed to a process for the production of corrugated boards employing the adhesive of U.S. Pat. No. 2,102,937.
U.S. Pat. No. 3,498,869 discloses a corrugated board adhesive consisting of an aqueous suspension of finely divided solid polyvinyl alcohol which is substantially insoluble in water at up to about 45.degree. C. and is readily soluble above about 80.degree. C. (i.e., at least 70 percent by weight of the polyvinyl alcohol will remain as unswollen solid at temperatures up to about 45.degree. C. and at least 80 percent by weight of the polyvinyl alcohol will quickly swell or dissolve in water at temperatures above about 80.degree. C.); various modifiers such as water soluble resins (for example, soluble starch, dextrin, sodium carboxymethyl cellulose, methoxycellulose, polyvinyl alcohol, their water soluble derivatives, etc.), or water soluble salts (for example, sodium sulfate, ammonium sulfate, sodium chloride, ammonium chloride, potassium chloride, calcium chloride or sodium acetate); extenders such as clay, kaolin, bentonite, silica, diatomaceous earth, calcium carbonate, alumina, starch, etc.; a gelling agent such as boric acid, borate, etc.; and, optionally, surface active agents and if wet adhesive strength is desired to be improved aminoplasts. Specific compositions disclosed comprise about 31 to 50 percent by weight of dry materials (i.e., excluding the water) of polyvinyl alcohol, about 44 to 56 percent clay, and from 0 to about 23 percent starch.